Riser apparatus assembly and method of installing same

ABSTRACT

A riser apparatus and method according to which an anchor is installed on the sea bed and a riser conduit is pivotally mounted on the anchor. The riser conduit is provided with end connectors for connection to an upper riser section leading, respectively, to a production platform and a lower riser section leading to a wellhead or subsea storage or export facility. The riser conduit is preformed into a predetermined curved shape and is provided with stiffening webs to safeguard its rigidity, so that it substantially retains its predetermined shape when movements are transmitted through the riser conduit from the upper riser section to the lower riser section, or conversely.

CROSS-REFERENCED TO RELATED APPLICATIONS

The present application claims priority to Great Britain application No. 0401837.0 filed Jan. 28, 2004.

BACKGROUND

This invention relates to riser apparatus, a riser assembly and a method of installing a riser assembly.

As is well known in the petroleum and petroleum services industries, a riser is essentially a pipe that is used for conveying hydrocarbon production fluids from a wellhead or subsea storage facility to a neighbouring production platform, from where it is transported by tanker or pipeline to a land-based facility for refining or other processing.

Risers are known in two principal forms, i.e. flexible risers and rigid risers. A flexible riser is a riser whose pipe wall is of multi-layered construction, which is capable of bending to accommodate changes in orientation of the pipe, in particular at the single ‘touchdown’ location on the sea bed where the pipe is redirected over a relatively sharply curved section of pipe from an inclined or substantially vertically downward orientation to a generally horizontal orientation, running on the sea bed. A flexible pipe offers the advantage of being able to accommodate a tighter bend radius at the touchdown location without risk of damage to the pipe wall. In addition, the flexible pipe is able to accommodate adjustment of the positioning of the upper end of the pipe at the sea surface relative to the portion of the riser that is lying on the sea bed, as well as any repositioning of the section lying on the sea floor. The main drawback of a flexible pipe is its relatively high cost due to its special, multi-layered, construction.

By contrast, a rigid pipe is ordinarily merely a metal-walled pipe, usually of a specially selected steel alloy. At the touchdown location, the pipe is deformed or bent so as to adopt a curved shape, but the degree of bending of the pipe has to be kept within the elastic limit of the steel used. As in the case of a flexible pipe, adjustments in the position of the upper end of the pipe relative to the section on the sea bed, and is vice versa, accommodated by the rigid pipe, in this case by elastic deformation of the pipe wall. Rigid pipes are considerably cheaper than flexible pipes, but the maximum curvature that can be accommodated at the touchdown region is considerably less than for a flexible riser and therefore special measures need to be taken, especially at the touchdown position, to guarantee that the riser is not deformed beyond its elastic limit.

Various techniques are known for anchoring a flexible or rigid riser at the touchdown location for different riser configurations. In addition to a free-hanging configuration in which the riser is suspended from a production platform as a free-hanging catenary which is reorientated at the touchdown location to run along the sea bed, various other configurations are known in which intermediate buoyancy support is given to the catenary at a location between the sea surface and the touchdown location and the riser reoriented at the touchdown location itself. Examples of such configurations, in particular Steep-S, Steep-Wave, Lazy-S and Lazy-Wave configurations are disclosed in the American Petroleum Institute Recommended Practice publication, 17B. Furthermore, an example of a so-called Pliant Wave configuration is disclosed in U.S. Pat. No. 4,906,137. Known anchoring arrangements for such configurations typically involve the use of a system of clamps and tethers, which are relatively complex in construction and difficult to install, or at least require time-consuming installation procedures.

The various forms mentioned of configuration for the riser, whether a flexible riser or a rigid riser, at the touchdown location require the provision of a specially designed anchor and guide arrangement, such as described above, to maintain the riser at the touchdown location while protecting it from excessive bending due to any adjustment of position of the ascending riser relative to the riser portion on the sea bed and the action of waves and currents, or vise versa.

It is now desired to provide a riser apparatus which is of simple construction, relatively cheap to make and install, and which provides high integrity for the riser while safeguarding it against damage due to inevitable adjustments in position that typically occur with an installed riser.

SUMMARY

According to the invention, there is provided riser apparatus comprising an anchor adapted to be installed on the sea bed and a riser conduit mounted on the anchor and preformed into a predetermined shape such that one end of the riser is orientated for connection to a first riser section on the sea bed and the other end of the riser conduit is differently orientated from said one end for connection to a second riser section leading to the surface of the sea, the riser conduit being substantially rigid such that it substantially retains its predetermined shape when, in use of the riser apparatus, adjustment of the relative positions of the first and second riser sections occurs.

It will be appreciated that this riser apparatus is extremely simple in construction, comprising essentially two main structural elements, that is the anchor and the riser conduit. The anchor provides the known function of anchoring the riser apparatus to the sea bed. The riser conduit provides the necessary reorientation for the completed riser so that the production hydrocarbon fluid can be directed from the sea bed location to the production platform. Since the riser conduit is substantially rigid and substantially retains its predetermined shape during use of the riser apparatus, it has no moving parts, which ensures operational reliability and helps to keep the cost of the riser apparatus low.

It will be further appreciated that the design philosophy avoids accommodating bending at the touchdown region where the principal reorientation of the riser takes place. Therefore, any change in position of the second riser section leading to the sea surface due to adjustment of the position of the supporting offshore platform or the like, or the effect of waves or currents on that section will be accommodated by flexing of the second riser section (and also of the first riser section if the rise conduit is adjustably mounted on the anchor). Since the riser apparatus will normally be installed in sea depths that require a relatively long length for the second riser section, the required adjustments can be accommodated by flexure even of a rigid upper riser section within the elastic limits of the steel from which that riser section is made. Alternatively, any adjustment in position of the first riser section relative to the second can be accommodated in corresponding fashion.

Preferably, the riser conduit comprises a curved length of conduit, rather than a conduit shaped so as to have sharp discontinuities in direction at one or more points along its length. Use of a curved conduit minimizes turbulence in the flow of production fluids and also enables conventional pigging techniques to be performed at locations between the curved riser conduit and the wellhead (or subsea storage facility), or downhole.

Preferably the riser conduit is provided with at least one stiffening web to contribute to the rigidity of the riser conduit. It is especially preferred that a pair of elongate stiffening webs be secured on the outside surface of the length of conduit, each such web extending in the direction of the flowpath axis of the length of conduit with one web located generally above the flowpath axis and the other web located generally below that axis. By arranging the webs with this orientation, maximum resistance to any bending of the conduit section within a vertical plane can be firmly resisted. Conveniently, such webs can be welded to the curved length of conduit, where the conduit is made of a suitable steel.

In a preferred embodiment, the riser conduit section is mounted on the anchor for pivoting about a generally horizontal axis. In this way, the riser apparatus can adapt better to relative movements between the first (lower) and second (upper) conduit sections, the riser conduit can then transfer some of this movement from one to the other conduit section. A particularly simple, effective and inexpensive form for such pivoting is afforded by linking the riser conduit to the anchor by respective interlocking shackles, or the like.

The anchor can take any form which is suitable for holding the riser apparatus in place on the sea bed. In principle, the anchor can be a structure designed to be weighted down with clump weights. However, one preferred form of anchor is one designed with sufficient rigidity that it can be driven into the seabed by pile or hammer driving. Another preferred form for the anchor is a suction anchor, i.e. one which relies on a suction effect to hold it in place on the sea bed. Such an anchor may be driven into the sea bed by evacuating an internal chamber so as to cause the hydrostatic pressure to apply a downward driving force on the anchor. Alternatively, if the sea bed formation is especially hard or rocky, pile driving may be required instead. In both cases, the anchor resists being withdrawn from the resultant pocket formed in the sea bed by a suction effect.

A preferred form of suction anchor is a rigid top structure with a downwardly depending cylindrical skirt adapted to be driven into the sea bed. The driving force can be produced by suction alone, but if this is not adequate to cause the skirt to penetrate the sea bed sufficiently, pile driving can be used. The integrity of anchoring is ensured by the reduced pressure inside the skirt, or by a vacuum created therein.

The two ends of the riser conduit can be provided with any form of end connections for the connection of respective riser sections to form a continuous riser running along the sea bed and then being reorientated up to the sea surface. It is particularly preferred that the riser conduit be provided at its two ends with end flanges for bolting to counterpart flanges on the ends of the first and second riser sections.

In order to assist, at the time of assembly, the lowering of an upper riser section for connection to the riser conduit of the riser apparatus, a sheave assembly, having a sheave pulley wheel, may be mounted on the anchor so that a pull down line may be passed from the sea surface around the pulley and be attached to a first riser section for pulling the first riser section down to the riser conduit for connection to said other end thereof.

It will be appreciated from the foregoing description that the present invention also provides a riser assembly comprising a riser apparatus in any one of the various forms described above, together with a first riser section on the sea bed connected at one end to said first end of the riser conduit, and a second riser section leading to the sea surface connected at one end to said second end of the riser conduit, whereby hydrocarbon production fluid may be directed from a sea bed location (such as a wellhead or subsea storage facility) to the sea surface successively through the first riser section, the riser conduit and the second riser section. The riser can also serve, conventionally, for conveying liquids from the surface down to the sea bed, for example for the supply of drilling fluid and the like during drilling operations, or serve as an export riser.

In one embodiment, at least the first riser section is a flexible conduit and bend restrictors are provided on the first riser section adjacent its one end. Such bend restrictors protect the first riser section at its one end that would otherwise be somewhat vulnerable to damage due to movements in one or the other riser section.

The riser assembly defined above can readily be installed by an installation method comprising the steps of (i) pre-installing the anchor of the riser apparatus on the sea bed, (ii) connecting said first and second riser sections to the riser conduit, (iii) lowering successively the first riser section and the second riser section so as to place the first riser section on the sea bed and position the riser conduit adjacent the pre-installed anchor, and (iv) mounting the riser conduit on the pre-installed anchor.

Such method can be relatively easily implemented in practice, using an attendant support vessel having a crane or other lowering equipment.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:

FIG. 1 is a side elevational view of one form of riser assembly in accordance with the present invention;

FIG. 2 is a general perspective view from above of the riser assembly; and

FIGS. 3 to 9 are diagrammatic representations of successive steps in a preferred method for installing the riser assembly.

DETAILED DESCRIPTION

Referring to FIGS. 1 and 2, the riser assembly, generally denoted by reference numeral 100, itself includes a riser apparatus 1 interconnecting an upper flexible riser section 8 leading to the sea surface in Steep-Wave configuration and a lower flexible riser section 9 lying on the sea bed.

The riser apparatus 1 comprises two principal elements, i.e. an anchor 2 and a riser conduit 3 mounted on the anchor in a manner to be described.

This anchor is designed as a suction anchor and comprises an upper frame 4 constructed from a plurality of girders 5 forming a grill. Secured to the upper frame 4 is a cylindrical skirt 6 which is closed at its upper end but open at its bottom end so that the anchor can be driven into the sea bed by evacuating the space within the skirt or, if the sea bed formation is particularly hard, a conventional technique such as pile-driving may be required to secure the anchor in place on the sea bed at the touchdown location for the riser to be installed.

The riser conduit 3 comprises a length of curved conduit which as shown consists of a multiplicity of conduit sections 3′ arranged end-to-end in a unitary assembly with a gradual change in directivity from each conduit section to the next. Alternatively, the curved conduit 3 could comprise a single length of conduit which is formed into an arcuate shape so that the fluid flow path through the conduit changes direction gradually and continuously and there are no sharp internal surface discontinuities which would otherwise enhance a turbulent flow of the production fluid or prevent conventional pigging operations beyond (i.e. well-side) of the riser conduit 3.

The metallurgy of the steel from which the curved conduit 3 is made and its wall thickness and design are such as to impart a high degree of rigidity to the conduit, such that it will substantially retain its predetermined shape, whenever either riser section 8, 9 moves and applies forces to the respective end of the riser conduit 3. However, to guarantee that the riser conduit 3 is not deformed or damaged under these circumstances, the riser conduit 3 is provided with at least one, and as shown two, stiffening webs 10 a, 10 b. As seen in FIGS. 1 and 2, these webs extend in the direction of the flowpath of the curved conduit 7, with the stiffening web 10 a located above the conduit flowpath and the other web 10 b located generally below that flowpath, so that each major surface of each web is arranged in a vertical plane. In this way, the stiffening webs 10, 10 b provide maximum resistance to bending of the riser conduit 3 about a generally horizontal axis.

As shown in the Figures, the upper and lower ends of conduit 3 are provided with upper and lower flange connectors 3 a, 3 b, respectively, which are bolted to counterpart end flanges 8′, 9′ of the upper and lower riser sections 8, 9, respectively so as to form the riser assembly 100 having a continuous riser.

Preferably as illustrated, the riser conduit 3 is mounted on the anchor 2 for pivoting about a generally horizontal axis, such mounting being provided by a first shackle 11 bolted to the riser conduit 3, specifically by means of a bolt secured to the lower stiffening web 10 b, interlinking with a second shackle 12 secured to the upper frame 4 in any suitable manner (e.g. by bolting). In this way, the rigid riser conduit 3 can transmit movements from the upper, rigid, riser section 8 to the lower, flexible, riser section 9 and vice versa.

For assisting the installation process to be described below, a sheave 13, including a sheave pulley wheel, is mounted on the upper frame 4 so that a line 14 can be passed down from an attendant support vessel, around the sheave wheel and be passed up to, and fixed by its free end to, the upper riser section 8. The function of the sheave assembly 13 and pull down line 14 will be described below.

The lower, flexible, riser section 9 on the sea bed is provided at its end adjacent the riser conduit 3 with a plurality of optional bend restrictors 14 to protect the pipe at this otherwise vulnerable end region from stressing.

It will be appreciated that although the described riser apparatus is of very simple construction, nevertheless it will be relatively massive, since it will be subjected to forces of the magnitude of several tons, even typically several tens of tons, when it is in use on the sea floor. Due to the very heavy weight of the individual components of the riser apparatus, it is preferred that the riser apparatus be installed, one principal component at a time, rather than as a fully assembled unit. A preferred assembly procedure will now be described with reference to FIGS. 3 to 9.

Firstly, the anchor 4 with fitted interlocking shackles 11, 12 and sheave assembly is lowered from the attendant support vessel to the touchdown location and the anchor is then installed on the sea bead 7, in a manner such as described above (see FIG. 3).

Then, as shown in this Figure, the upper, flexible, riser section 8, having a buoyancy section 21, and the lower, flexible, riser section 9 are connected to the corresponding ends of the riser conduit 3 and a load transfer rigging or line 19 is connected to respective points on the lower and upper riser sections that are spaced a short distance from their respective ends connected to the riser conduit 3 (as best shown in FIG. 8).

A tensioner or similar device on the vessel 15 is used to lower, successively, the lower riser section 9, the load transfer rigging 19, and the upper riser section 8 from attendant vessel 15. FIG. 3 shows the situation in which the riser conduit 3 with the lower riser section 9 suspended from it is being lowered from the attendant vessel 15, the riser conduit 3 being suspended from the end of the upper riser section 8. A clump weight 18 attached to the lower end section of the upper riser section 8 assists the lowering procedure and also helps to keep the interconnected components 8, 3, 9 aligned below the vessel 15, to facilitate their accurate positioning relative to the touch-down location.

Next, an initiation wire 16 suspended from the bottom of lower riser section 9 is connected to an initiation point 17 located on the sea bed 7 (FIG. 4) at a predetermined lateral offset relative to the installed anchor 4, and the vessel then moves forwardly to progressively lower the riser conduit 3 while displacing it forwardly (i.e. to the left in the Figures) of the initiation point (FIGS. 5 and 6), until the clump weight 18 comes to rest on the sea bed 7 (see FIG. 7). At this point, the lower riser section 9 is laid on the sea bed and the riser conduit 3 is positioned adjacent to the pre installed anchor 4. The riser conduit 3 is then temporarily connected to the anchor 4 by means of the interconnected shackles 11, 12.

To complete the lowering of the riser conduit 3, a pull down line 14 is passed down from the vessel, around the sheave pulley (see FIGS. 1 and 2), and attached to the upper riser section 8. By applying a traction force to the line 14 on the vessel, the upper riser section can be pulled down until the riser conduit 3 can be permanently connected to the anchor by disconnecting the temporary connection and then bolting the shackle 11 to the flange 10 b, to form a permanent connection.

Next, the upper riser section 8 is paid out further and deployed to the configuration shown in FIG. 9 in which the buoyancy section 21 on the upper riser section 8 holds that riser section in a steep wave configuration.

Lastly, the far end of the lower, flexible, riser section 9 is deployed and connected to the required wellhead or subsea storage or exit facility 23 FIGS. 3-7), while the upper end of the upper riser section 8 is deployed and connected to the appropriate equipment on a production platform 22 (FIG. 9) provided for the associated well, to complete the riser installation.

All subsea installation and assembly operations described above can be performed using drivers or a remotely operated vehicle (ROV), as appropriate.

Although the description given above refers only to a single sheave assembly 13, depending on the circumstances it may be preferred to provide a second sheave assembly 13 as shown, so that the pull-down operation for the upper, flexible, riser section 8 can be achieved by applying traction to two pull-down lines, which share the traction effort and apply the traction force equally.

It will be appreciated from FIG. 1 that the two stiffening webs 10 a, 10 b together form a stiffening web arrangement of “delta” shape, which is particularly advantageous in that it provides the required degree of stiffening with optimized (minimal) use of sheet metal, while the apex of the delta provides a suitably located and strong anchoring point for the change link 11.

Since the curvature of the riser conduit 3 is kept substantially fixed due to the substantially rigid construction of the riser conduit 3, no additional measures need to be taken to ensure that the riser conduit 3 is not damaged by the application of bending forces in use of the riser assembly. However, it is of course desirable that the curvature be “pigging friendly” so that pigging operations may take place well-side of the riser conduit 3. In practical terms, this requires that the minimum radius of curvature of the curved conduit 3 be at least five times the outside diameter of that conduit.

It will be appreciated that the disclosed riser apparatus, consisting of the anchor 2 and riser conduit 3 mounted thereon, is a constructionally simple piece of apparatus which can safely accommodate global adjustments of the positioning of the completed riser assembly without risk of damage to the curved conduit 7 at the touchdown location. In particular, it avoids the use of a flexible riser, continuous over its length along the sea bed to the touchdown region and up to the production platform, where the continuous flexible riser is typically anchored, as in the prior art, through a system of clamps and tethers, or the like. Instead, the disclosed riser apparatus and riser assembly achieve anchoring at a single point, where pivoting can be provided to accommodate global responses without the riser itself being subjected to bending deformations at that point.

In the disclosed embodiment, the upper and lower riser sections 8, 9 are flexible riser sections, which is preferred in order to accommodate deployment of these riser sections during assembly of the riser and subsequent movements or adjustments in position during subsequent normal operations. However, it will be appreciated that a rigid pipe can be used in place of one or each of these two riser sections 8, 9, where the or each rigid riser section is able to accommodate the anticipated displacements within their elastic limits.

Furthermore, although the upper riser section 8 is shown in Steep-Wave configuration, other orientations are of course possible. In all cases, the angle of re-orientation provided by the curved riser needs to be selected according to the needs of the riser configuration required. 

1. A riser apparatus comprising: an anchor adapted to be installed on the sea bed: a riser conduit connected to the anchor; a first riser section located on the sea bed and connected to one end of the riser conduit; and a second riser section connected to the other end of the riser conduit and extending to the surface of the sea; the riser conduit being preformed to permit the connections to the first and second riser sections and being substantially rigid such that it substantially retains its preformed shape when changes of the relative positions of the first and second riser sections occurs.
 2. The riser apparatus according to claim 1, wherein the riser conduit comprises a curved length of conduit.
 3. The riser apparatus according to claim 1 or 2, wherein the riser conduit is provided with at least one stiffening web to contribute to the rigidity of the riser conduit.
 4. The riser apparatus according to claim 3, wherein a pair of elongate stiffening webs is secured on the outside surface of the length of conduit, each such web extending in the direction of the flowpath axis of the length of conduit with one web located generally above the flowpath axis and the other web located generally below the flowpath axis.
 5. The riser apparatus according to claim 1, wherein the riser conduit is pivotally mounted to the anchor for pivoting about a generally horizontal axis.
 6. The riser apparatus according to claim 6, further comprising interlocking shackles for mounting the riser conduit to the anchor.
 7. The riser apparatus according to claim 1, wherein the anchor comprises a suction anchor.
 8. The riser apparatus according to claim 1, wherein the anchor is designed to be driven into the sea bed by pile driving.
 9. The riser apparatus according to claim 7, wherein the suction anchor comprises a rigid top structure with a downwardly depending cylindrical skirt adapted to be driven into the sea bed.
 10. The riser apparatus according to claim 1, wherein the riser conduit is provided at its two ends with end flanges for bolting to counterpart flanges on the ends of the first and second riser sections.
 11. The riser apparatus according to claim 1, further comprising a pulley wheel mounted on the anchor so that a pulldown line may be passed from the sea surface around the pulley and be attached to the second riser section for pulling the second riser section down so that the riser conduit can be connected to the anchor.
 12. The riser apparatus according to claim 1 wherein hydrocarbon production fluid may be directed from a sea bed location to the sea surface successively through the first riser section, the riser conduit and the second riser section.
 13. A riser assembly according to claim 1 wherein at least the first riser section is a flexible conduit and further comprises bend restrictors provided on the first riser section.
 14. A method of installing a riser assembly relative to a sea bed, the method comprising: pre-installing an anchor on the sea bed; connecting first and second riser sections to the riser conduit; lowering successively the first riser section and the second riser section so as to place the first riser section on the sea bed and position the riser conduit adjacent the pre-installed anchor; and connecting the riser conduit to the pre-installed anchor. 